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package java.nio.channels;

import java.io.IOException;
import java.nio.channels.spi.AbstractInterruptibleChannel;
import java.nio.channels.spi.SelectorProvider;


/**
 * A channel that can be multiplexed via a {@link Selector}.
 *
 * <p> In order to be used with a selector, an instance of this class must
 * first be <i>registered</i> via the {@link #register(Selector, int, Object)
 * register} method.  This method returns a new {@link SelectionKey} object
 * that represents the channel's registration with the selector.
 *
 * <p> Once registered with a selector, a channel remains registered until it
 * is <i>deregistered</i>.  This involves deallocating whatever resources were
 * allocated to the channel by the selector.
 *
 * <p> A channel cannot be deregistered directly; instead, the key representing
 * its registration must be <i>cancelled</i>.  Cancelling a key requests that
 * the channel be deregistered during the selector's next selection operation.
 * A key may be cancelled explicitly by invoking its {@link
 * SelectionKey#cancel() cancel} method.  All of a channel's keys are cancelled
 * implicitly when the channel is closed, whether by invoking its {@link
 * Channel#close close} method or by interrupting a thread blocked in an I/O
 * operation upon the channel.
 *
 * <p> If the selector itself is closed then the channel will be deregistered,
 * and the key representing its registration will be invalidated, without
 * further delay.
 *
 * <p> A channel may be registered at most once with any particular selector.
 *
 * <p> Whether or not a channel is registered with one or more selectors may be
 * determined by invoking the {@link #isRegistered isRegistered} method.
 *
 * <p> Selectable channels are safe for use by multiple concurrent
 * threads. </p>
 *
 *
 * <a name="bm"></a>
 * <h2>Blocking mode</h2>
 *
 * A selectable channel is either in <i>blocking</i> mode or in
 * <i>non-blocking</i> mode.  In blocking mode, every I/O operation invoked
 * upon the channel will block until it completes.  In non-blocking mode an I/O
 * operation will never block and may transfer fewer bytes than were requested
 * or possibly no bytes at all.  The blocking mode of a selectable channel may
 * be determined by invoking its {@link #isBlocking isBlocking} method.
 *
 * <p> Newly-created selectable channels are always in blocking mode.
 * Non-blocking mode is most useful in conjunction with selector-based
 * multiplexing.  A channel must be placed into non-blocking mode before being
 * registered with a selector, and may not be returned to blocking mode until
 * it has been deregistered.
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @see SelectionKey
 * @see Selector
 * @since 1.4
 */

public abstract class SelectableChannel
    extends AbstractInterruptibleChannel
    implements Channel {

  /**
   * Initializes a new instance of this class.
   */
  protected SelectableChannel() {
  }

  /**
   * Returns the provider that created this channel.
   *
   * @return The provider that created this channel
   */
  public abstract SelectorProvider provider();

  /**
   * Returns an <a href="SelectionKey.html#opsets">operation set</a>
   * identifying this channel's supported operations.  The bits that are set
   * in this integer value denote exactly the operations that are valid for
   * this channel.  This method always returns the same value for a given
   * concrete channel class.
   *
   * @return The valid-operation set
   */
  public abstract int validOps();

  // Internal state:
  //   keySet, may be empty but is never null, typ. a tiny array
  //   boolean isRegistered, protected by key set
  //   regLock, lock object to prevent duplicate registrations
  //   boolean isBlocking, protected by regLock

  /**
   * Tells whether or not this channel is currently registered with any
   * selectors.  A newly-created channel is not registered.
   *
   * <p> Due to the inherent delay between key cancellation and channel
   * deregistration, a channel may remain registered for some time after all
   * of its keys have been cancelled.  A channel may also remain registered
   * for some time after it is closed.  </p>
   *
   * @return <tt>true</tt> if, and only if, this channel is registered
   */
  public abstract boolean isRegistered();
  //
  // sync(keySet) { return isRegistered; }

  /**
   * Retrieves the key representing the channel's registration with the given
   * selector.
   *
   * @param sel The selector
   * @return The key returned when this channel was last registered with the given selector, or
   * <tt>null</tt> if this channel is not currently registered with that selector
   */
  public abstract SelectionKey keyFor(Selector sel);
  //
  // sync(keySet) { return findKey(sel); }

  /**
   * Registers this channel with the given selector, returning a selection
   * key.
   *
   * <p> If this channel is currently registered with the given selector then
   * the selection key representing that registration is returned.  The key's
   * interest set will have been changed to <tt>ops</tt>, as if by invoking
   * the {@link SelectionKey#interestOps(int) interestOps(int)} method.  If
   * the <tt>att</tt> argument is not <tt>null</tt> then the key's attachment
   * will have been set to that value.  A {@link CancelledKeyException} will
   * be thrown if the key has already been cancelled.
   *
   * <p> Otherwise this channel has not yet been registered with the given
   * selector, so it is registered and the resulting new key is returned.
   * The key's initial interest set will be <tt>ops</tt> and its attachment
   * will be <tt>att</tt>.
   *
   * <p> This method may be invoked at any time.  If this method is invoked
   * while another invocation of this method or of the {@link
   * #configureBlocking(boolean) configureBlocking} method is in progress
   * then it will first block until the other operation is complete.  This
   * method will then synchronize on the selector's key set and therefore may
   * block if invoked concurrently with another registration or selection
   * operation involving the same selector. </p>
   *
   * <p> If this channel is closed while this operation is in progress then
   * the key returned by this method will have been cancelled and will
   * therefore be invalid. </p>
   *
   * @param sel The selector with which this channel is to be registered
   * @param ops The interest set for the resulting key
   * @param att The attachment for the resulting key; may be <tt>null</tt>
   * @return A key representing the registration of this channel with the given selector
   * @throws ClosedChannelException If this channel is closed
   * @throws ClosedSelectorException If the selector is closed
   * @throws IllegalBlockingModeException If this channel is in blocking mode
   * @throws IllegalSelectorException If this channel was not created by the same provider as the
   * given selector
   * @throws CancelledKeyException If this channel is currently registered with the given selector
   * but the corresponding key has already been cancelled
   * @throws IllegalArgumentException If a bit in the <tt>ops</tt> set does not correspond to an
   * operation that is supported by this channel, that is, if {@code set & ~validOps() != 0}
   */
  public abstract SelectionKey register(Selector sel, int ops, Object att)
      throws ClosedChannelException;
  //
  // sync(regLock) {
  //   sync(keySet) { look for selector }
  //   if (channel found) { set interest ops -- may block in selector;
  //                        return key; }
  //   create new key -- may block somewhere in selector;
  //   sync(keySet) { add key; }
  //   attach(attachment);
  //   return key;
  // }

  /**
   * Registers this channel with the given selector, returning a selection
   * key.
   *
   * <p> An invocation of this convenience method of the form
   *
   * <blockquote><tt>sc.register(sel, ops)</tt></blockquote>
   *
   * behaves in exactly the same way as the invocation
   *
   * <blockquote><tt>sc.{@link
   * #register(java.nio.channels.Selector, int, java.lang.Object)
   * register}(sel, ops, null)</tt></blockquote>
   *
   * @param sel The selector with which this channel is to be registered
   * @param ops The interest set for the resulting key
   * @return A key representing the registration of this channel with the given selector
   * @throws ClosedChannelException If this channel is closed
   * @throws ClosedSelectorException If the selector is closed
   * @throws IllegalBlockingModeException If this channel is in blocking mode
   * @throws IllegalSelectorException If this channel was not created by the same provider as the
   * given selector
   * @throws CancelledKeyException If this channel is currently registered with the given selector
   * but the corresponding key has already been cancelled
   * @throws IllegalArgumentException If a bit in <tt>ops</tt> does not correspond to an operation
   * that is supported by this channel, that is, if {@code set & ~validOps() != 0}
   */
  public final SelectionKey register(Selector sel, int ops)
      throws ClosedChannelException {
    return register(sel, ops, null);
  }

  /**
   * Adjusts this channel's blocking mode.
   *
   * <p> If this channel is registered with one or more selectors then an
   * attempt to place it into blocking mode will cause an {@link
   * IllegalBlockingModeException} to be thrown.
   *
   * <p> This method may be invoked at any time.  The new blocking mode will
   * only affect I/O operations that are initiated after this method returns.
   * For some implementations this may require blocking until all pending I/O
   * operations are complete.
   *
   * <p> If this method is invoked while another invocation of this method or
   * of the {@link #register(Selector, int) register} method is in progress
   * then it will first block until the other operation is complete. </p>
   *
   * @param block If <tt>true</tt> then this channel will be placed in blocking mode; if
   * <tt>false</tt> then it will be placed non-blocking mode
   * @return This selectable channel
   * @throws ClosedChannelException If this channel is closed
   * @throws IllegalBlockingModeException If <tt>block</tt> is <tt>true</tt> and this channel is
   * registered with one or more selectors
   * @throws IOException If an I/O error occurs
   */
  public abstract SelectableChannel configureBlocking(boolean block)
      throws IOException;
  //
  // sync(regLock) {
  //   sync(keySet) { throw IBME if block && isRegistered; }
  //   change mode;
  // }

  /**
   * Tells whether or not every I/O operation on this channel will block
   * until it completes.  A newly-created channel is always in blocking mode.
   *
   * <p> If this channel is closed then the value returned by this method is
   * not specified. </p>
   *
   * @return <tt>true</tt> if, and only if, this channel is in blocking mode
   */
  public abstract boolean isBlocking();

  /**
   * Retrieves the object upon which the {@link #configureBlocking
   * configureBlocking} and {@link #register register} methods synchronize.
   * This is often useful in the implementation of adaptors that require a
   * specific blocking mode to be maintained for a short period of time.
   *
   * @return The blocking-mode lock object
   */
  public abstract Object blockingLock();

}
